Compact lamps are widely used for producing electric light. In comparison with incandescent bulbs, these lamps are characterized by a higher lighting efficiency and a longer useful life. Compact lamps produce light through a process of gas discharge in a bent discharge vessel.
With known compact lamps, the gas discharge is accomplished by aligning two or more U-shaped tubes in sequence, so that two U-tubes are linked to each other via a hollow connection point at one of their legs (hot-kiss process). There are many disadvantages to this technique.
To begin with, the production of U-tubes presents its own problems. U-tubes are usually produced by bending rod-shaped glass tubes which are open at both ends. This leads unavoidably to an irregular thickness of the walls of the U-tube where it is bent. In particular, the uneven distribution of glass material in the bends produces stresses in the glass, which make it necessary to use expensive handling techniques during the subsequent processing stages, and which represent a prime cause of lamp failure.
Furthermore, a rapid change occurs at the hollow connectors in the strength of the electric field of the positive gas-discharge column. To obtain high efficiency with a compact lamp, however, the strength of the electric field must be constant and steady throughout the length of the positive column. Variations in the strength of the electric field at the hollow connection points cause a reduction in the light output of a compact lamp.
An electric discharge lamp is disclosed in DE OS 30 44 058 using a multiple-fold discharge tube. The U-shaped portions of the discharge tube are produced by bending rod-shaped glass tubes, with the result that the thicknesses of the walls of the discharge tube in these portions are naturally uneven. Although it is recommended to overcome such deformations by injecting a stream of inert gas under pressure into the discharge tube, this step nevertheless can at best correct the diameter of the discharge tube, and not the uneven distribution of glass within the bends.
A process for producing a U-tube is described in GB-PS 668 259, in which two glass tubes, arranged parallel to each other, are heated at their lower and opposing ends. The heated tube ends are connected together mechanically at their opposing sides and are opened lengthwise at their inner sides along a section that corresponds approximately to twice the diameter of the glass tube. The tube ends are then heated using laterally arranged burners, to produce a glass vessel with an oval opening on its lower side.
The cross-sectionally oval opening in the glass vessel is closed by using two opposing burners arranged on either side of the glass vessel to heat the glass externally to its melting point, at a right angle to the longitudinal axis of the glass tube, along a narrow horizontal line. This horizontal line runs at some vertical distance from the opening of the glass tube that corresponds to the glass tube diameter. The glass is then sheared off beneath this horizontal line, and the glass vessel is sealed, leaving a seam. Alternatively, the glass beneath the horizontal line is simply melted by the lateral burners, and the glass so removed collects in a drop and falls to the floor. In this case the glass vessel is sealed by the dripping action of the remnant of glass. The sealed vessel is then shaped by blowing inside a mould.
The particular disadvantage of this process is that the leaving of a glass remnant means a shortening of the glass tube. In this connection, it is a disadvantage of known process that the opposing glass tubes must be opened lengthwise along a section that corresponds to approximately twice the glass tube diameter.
It should be noted that it is not possible, using multiple applications of the known process, to produce a folded single-tube glass vessel so that the two open tube ends lie adjacent to each other. Multiple applications of the known process only make it possible to produce an elongated single-tube glass vessel that is folded along a straight line.
This is related to the fact that with the known process, a glass remnant is removed each time by laterally located burners. Burners must therefore be arranged in a horizontal relation to both sides of the oval-aperture vessels. This however is basically impossible, in cases where the individual sections of a single-tube glass vessel are conducted back and forth, as is necessary if the two open tube ends of a single-tube glass vessel are to lie beside each other. In such cases the area of glass to be processed will be covered up by the presence of other portions of the glass tube, at least on one side of an oval-aperture vessel, so that the laterally arranged burners cannot heat the appropriate area of glass, and the known process fails.